_DCB structure

Defines the control setting for a serial communications device.

Syntax

typedef struct _DCB {
  DWORD DCBlength;
  DWORD BaudRate;
  DWORD fBinary : 1;
  DWORD fParity : 1;
  DWORD fOutxCtsFlow : 1;
  DWORD fOutxDsrFlow : 1;
  DWORD fDtrControl : 2;
  DWORD fDsrSensitivity : 1;
  DWORD fTXContinueOnXoff : 1;
  DWORD fOutX : 1;
  DWORD fInX : 1;
  DWORD fErrorChar : 1;
  DWORD fNull : 1;
  DWORD fRtsControl : 2;
  DWORD fAbortOnError : 1;
  DWORD fDummy2 : 17;
  WORD  wReserved;
  WORD  XonLim;
  WORD  XoffLim;
  BYTE  ByteSize;
  BYTE  Parity;
  BYTE  StopBits;
  char  XonChar;
  char  XoffChar;
  char  ErrorChar;
  char  EofChar;
  char  EvtChar;
  WORD  wReserved1;
} DCB, *LPDCB;

Members

DCBlength

The length of the structure, in bytes. The caller must set this member to sizeof(DCB).

BaudRate

The baud rate at which the communications device operates. This member can be an actual baud rate value, or one of the following indexes.

Value Meaning
CBR_110
110
110 bps
CBR_300
300
300 bps
CBR_600
600
600 bps
CBR_1200
1200
1200 bps
CBR_2400
2400
2400 bps
CBR_4800
4800
4800 bps
CBR_9600
9600
9600 bps
CBR_14400
14400
14400 bps
CBR_19200
19200
19200 bps
CBR_38400
38400
38400 bps
CBR_57600
57600
57600 bps
CBR_115200
115200
115200 bps
CBR_128000
128000
128000 bps
CBR_256000
256000
256000 bps

fBinary

If this member is TRUE, binary mode is enabled. Windows does not support nonbinary mode transfers, so this member must be TRUE.

fParity

If this member is TRUE, parity checking is performed and errors are reported.

fOutxCtsFlow

If this member is TRUE, the CTS (clear-to-send) signal is monitored for output flow control. If this member is TRUE and CTS is turned off, output is suspended until CTS is sent again.

fOutxDsrFlow

If this member is TRUE, the DSR (data-set-ready) signal is monitored for output flow control. If this member is TRUE and DSR is turned off, output is suspended until DSR is sent again.

fDtrControl

The DTR (data-terminal-ready) flow control. This member can be one of the following values.

Value Meaning
DTR_CONTROL_DISABLE
0x00
Disables the DTR line when the device is opened and leaves it disabled.
DTR_CONTROL_ENABLE
0x01
Enables the DTR line when the device is opened and leaves it on.
DTR_CONTROL_HANDSHAKE
0x02
Enables DTR handshaking. If handshaking is enabled, it is an error for the application to adjust the line by using the EscapeCommFunction function.

fDsrSensitivity

If this member is TRUE, the communications driver is sensitive to the state of the DSR signal. The driver ignores any bytes received, unless the DSR modem input line is high.

fTXContinueOnXoff

If this member is TRUE, transmission continues after the input buffer has come within XoffLim bytes of being full and the driver has transmitted the XoffChar character to stop receiving bytes. If this member is FALSE, transmission does not continue until the input buffer is within XonLim bytes of being empty and the driver has transmitted the XonChar character to resume reception.

fOutX

Indicates whether XON/XOFF flow control is used during transmission. If this member is TRUE, transmission stops when the XoffChar character is received and starts again when the XonChar character is received.

fInX

Indicates whether XON/XOFF flow control is used during reception. If this member is TRUE, the XoffChar character is sent when the input buffer comes within XoffLim bytes of being full, and the XonChar character is sent when the input buffer comes within XonLim bytes of being empty.

fErrorChar

Indicates whether bytes received with parity errors are replaced with the character specified by the ErrorChar member. If this member is TRUE and the fParity member is TRUE, replacement occurs.

fNull

If this member is TRUE, null bytes are discarded when received.

fRtsControl

The RTS (request-to-send) flow control. This member can be one of the following values.

Value Meaning
RTS_CONTROL_DISABLE
0x00
Disables the RTS line when the device is opened and leaves it disabled.
RTS_CONTROL_ENABLE
0x01
Enables the RTS line when the device is opened and leaves it on.
RTS_CONTROL_HANDSHAKE
0x02
Enables RTS handshaking. The driver raises the RTS line when the "type-ahead" (input) buffer is less than one-half full and lowers the RTS line when the buffer is more than three-quarters full. If handshaking is enabled, it is an error for the application to adjust the line by using the EscapeCommFunction function.
RTS_CONTROL_TOGGLE
0x03
Specifies that the RTS line will be high if bytes are available for transmission. After all buffered bytes have been sent, the RTS line will be low.

fAbortOnError

If this member is TRUE, the driver terminates all read and write operations with an error status if an error occurs. The driver will not accept any further communications operations until the application has acknowledged the error by calling the ClearCommError function.

fDummy2

Reserved; do not use.

wReserved

Reserved; must be zero.

XonLim

The minimum number of bytes in use allowed in the input buffer before flow control is activated to allow transmission by the sender. This assumes that either XON/XOFF, RTS, or DTR input flow control is specified in the fInX, fRtsControl, or fDtrControl members.

XoffLim

The minimum number of free bytes allowed in the input buffer before flow control is activated to inhibit the sender. Note that the sender may transmit characters after the flow control signal has been activated, so this value should never be zero. This assumes that either XON/XOFF, RTS, or DTR input flow control is specified in the fInX, fRtsControl, or fDtrControl members. The maximum number of bytes in use allowed is calculated by subtracting this value from the size, in bytes, of the input buffer.

ByteSize

The number of bits in the bytes transmitted and received.

Parity

The parity scheme to be used. This member can be one of the following values.

Value Meaning
EVENPARITY
2
Even parity.
MARKPARITY
3
Mark parity.
NOPARITY
0
No parity.
ODDPARITY
1
Odd parity.
SPACEPARITY
4
Space parity.

StopBits

The number of stop bits to be used. This member can be one of the following values.

Value Meaning
ONESTOPBIT
0
1 stop bit.
ONE5STOPBITS
1
1.5 stop bits.
TWOSTOPBITS
2
2 stop bits.

XonChar

The value of the XON character for both transmission and reception.

XoffChar

The value of the XOFF character for both transmission and reception.

ErrorChar

The value of the character used to replace bytes received with a parity error.

EofChar

The value of the character used to signal the end of data.

EvtChar

The value of the character used to signal an event.

wReserved1

Reserved; do not use.

Remarks

When a DCB structure is used to configure the 8250, the following restrictions apply to the values specified for the ByteSize and StopBits members:

  • The number of data bits must be 5 to 8 bits.
  • The use of 5 data bits with 2 stop bits is an invalid combination, as is 6, 7, or 8 data bits with 1.5 stop bits.

Requirements

   
Minimum supported client Windows XP
Minimum supported server Windows Server 2003
Header winbase.h (include Windows.h)

See Also

BuildCommDCB

ClearCommError

EscapeCommFunction

GetCommState

SetCommState